23 July 2010

The perils of cheap adaptors II

The "T" adaptor in the photo was the reason a WACOM WP-639 duplexer cavity couldn't achieve a notch of more than about 12dB. As you may notice, there is a small helicoidal spring "joining" the two center conductor members in the adaptor. Being a small inductor, you may imagine what effect the presence of this spring has on VHF signals!
This is quite evident in the SA-TG screen photo. The response dip (notch depth) should have been about 35dB, but only 12 dB could be achieved, because of the impedance and loss of the spring, on the signal's way to the resonant cavity.
An "Amphenol" adaptor cleared the problem immediately and restored the notch depth to 35dB.

This unacceptable adaptor was the reason the duplexer had to be hauled down from the mountain and retuned. Even on HF, this kind of connection might create problems with its reactance - to say nothing of its deplorable reliability.

Do you have any such garbage in your VHF-UHF setup? 

If the captions in the photos all seem Greek to you, that's because they indeed ARE Greek! (Well, mostly!)

19 July 2010

The perils of cheap adaptors

Cheap RF connectors / adaptors may not only "damage" your signal, but also your rig! Take a look at a female type-N connector that suffered distortion of the center contact because of a cheap, unsuitable adaptor - the center contact is completely bent-up!
Fortunately, this specific connector could be brought back in shape, but I have seen others that had to be replaced because the contact "fingers" had broken or had bent in such a way that they couldn't be saved...

Remember, you only pay for a good adaptor or connector once!

It's getting hotter...

In a previous post I discussed the problem of imperfect contact between the power amplifier modules and the heat sink in an TM-D710. Last weekend I worked on three more D-710s, which had developed the infamous "withering filter" illness, and discovered that the imperfect contact matter may reach extreme proportions!

Take a look at the photos (click on them to enlarge). The power modules barely made contact with the heat sink, only near the affixing screws. Less than 20% of the surface was in contact! Not a very good scenario for the longevity of the modules!
The other two rigs showed about 25% and 10% loss of contact, and only at one of their two modules, respectively.  The photos are of the "worst case" rig. As an afterthought, I should have tried to see which is at fault, the modules or the heat sink, but I foolishly didn't do that. I will in the next transceiver that comes along, and let you know. My wild guess is that the die-cast heat sink machine finishing (leveling) at that spot is at fault; but  I am certainly not sure.

What to do if your rig suffers this way? A couple of drops of gear oil will probably slide your problems away and cool things off, see the older post.

12 July 2010

The Mysterious Case of the Withering Filters

The ceramic filters found in almost all  radio communications equipment are indeed extremely useful components. Small, cheap and efficient, those "little black boxes" have found widespread use by all the manufacturers. I used to think that they're almost indestructible, because I had never seen one of them fail - but this fact has changed.
I have recently come across several cases of VHF-UHF transceivers where the receiver suddenly went deaf, faintly hearing signals only above -60dBm. The culprit was the ceramic 2nd IF filter (450 or 455 kHz) in all those cases. All those filters I have examined showed the same symptoms: the output side showed a low (a few tens of ohms) resistance to ground for DC, where it should have been almost infinite resistance.
After a while (and having replaced several such filters in my friends' transceivers), I became curious and investigated the reasons why those fairly robust components had become bad.

First, let's talk a bit about their structure (see the diagram I made, click on it to enlarge). Most of the 6-pole ceramic filters used in amateur radio transceivers have the general structure shown on the left. There are six ceramic (barium titanate, if memory serves) resonator elements, three in series and three shunt, connected as shown. The series elements are thick, the shunt elements are thin, and both have their wide surfaces plated with a metal (I guess it's a silver alloy). The narrow edge surfaces are not plated.
There are metal inserts between the ceramic elements, making contact to the plated surfaces of the elements and providing the electrical connections to the outside world. The whole structure is housed in a small plastic case, which is hermetically sealed with epoxy resin at the bottom side, where the pins come out.

So, what was the problem? While waiting for a replacement filter for a rig, I decided to try to pry open the case of the failed filter. I did so with the edge of a very sharp X-acto cutter, and I carefully removed the black case. One of the ceramic resonators fell off, along with a (phosphor bronze?) tensioner spring plate, which keeps everything pressed together when the filter is in its case.
It was immediately evident that something was wrong, because the spring plate was visibly oxidised, and there were suspicious looking spots at the edges of the thin ceramic element (you may notice one of them just below the right corner). I measured with my ohmmeter and saw that that element was the bad one, because the resistance reading was 19 ohms, the same value I had previously measured between the output pin and ground. If you take a good look at the next macro photo, you will see that the corners of the other elements also have low-resistance deposited paths short-circuiting the elements in the same way (also, look at the lower part of the diagram). To my surprise, with a magnifying glass I observed tiny droplets of a clear liquid (water, I think) at the inside walls of the case! What was the story here?

Hello, electromigration! The "crime scene" had all the necessary elements required for electromigration to do its nasty stuff. But let's take things in order.
a) Ceramic filter manufacturers (ALL of them!) expressly warn against applying a DC voltage at the input and output pins of the filters. Why? The reason is electromigration!
b) Electromigration is a process where, under the influence of an electric field and in the slightest presence of moisture, metal (especially silver) starts migrating and forming conductive paths (called dendrites, from their tree-like appearance, δένδρον [dendron] in Greek) across insulating materials. This phenomenon is a major headache e.g. for IC manufacturers, significantly lowering the reliability of their products.
c) The final result in our case is that (especially across the thin ceramic element edges) conductive paths of metal (and oxides from the electrolytic process since moisture exists inside the filter) are formed, short-circuiting it. Good-bye, filter?

Don't fret, there is still hope! (if you're good at handling very small parts - that's the catch). I thought that if I could get the elements out of the structure one by one and clean their edges, thus eliminating the conductive path, perhaps the filter would work again. That's very easy, because they aren't soldered in place, they just get "clamped" between pairs of contacts when the filter case is in place. If you decide to do that, only get ONE element out at a time with a pair of needle-nose tweezers, clean all of its its narrow edges by wiping them lightly across very fine grit sandpaper a couple of times and then replace it exactly where and how it was - don't mix them up! Also, don't touch the resonator elements with your hands, finger oils will contaminate them and possibly change their resonant frequency! Carefully clean oxidation wherever you can spot it by scraping, always being careful not to spill the guts of the filter! If you do spill them, they can be put back in place IF you have taken notes and photos of the filter's structure. I cleaned and dried the interior of the case, too. Before putting each resonator back in its place, check with your ohm-meter, you should get an infinite resistance reading - anything else indicates you need to repeat the cleaning process - gently!
Finally, I put the filter back together, sealed it with a minute quantity of cyanoacrylate and soldered it back in the transceiver. Lo and behold, the receiver sprang back to life - and at full specified sensitivity, as my measurements showed. The pass-band response hasn't changed. I think that now, after my delicate sandpaper treatment, the filter is a lot less possible to again fall victim to the nasty electromigration, because all the edges are quite clean now, there isn't any metal there any more.Time will show!

The final word: Ceramic filter manufacturers are quite right in warning against applying DC voltages at the input / output pins of the filters. They specify the use of a DC blocking capacitor at the filter's input and especially the output. Application of DC voltage causes electromigration and corrosion to initiate (especially in humid environments where temperature variations eventually promote water vapor condensation inside the filter, which may have imperfect sealing), and after a period of time the filter fails in the way we discussed.

The funny thing is, most of the manufacturers of amateur radio (and commercial) transceivers amazingly and inexplicably DON'T use the blocking capacitors, instead they boldly apply DC potentials directly at the filter's pins. A survey of several schematic diagrams confirmed this, especially in transceivers where there are several ceramic filters switched in and out of the signal path with diodes  and DC bias (usually about 8V). Why they do so beats me, perhaps it is to save some cents for a pair of blocking capacitors for each filter, creating a huge reliability problem on the way...

The enterprising radio amateur can always add those capacitors in the circuit (0.1 μF, 50V, 0603 size SMD ceramic capacitors are great) and save her / his receiver from becoming deaf due to a ...withering filter! Admittedly, this is a bit difficult in most modern rigs, due mainly to the small dimensions of the components and layout... but it's certainly worth a try.

***ADDENDUM: For those who want to learn more about the phenomenon that I propose that causes DC-biased filter failure, please take a look here: http://www.ami.ac.uk/courses/topics/0158_emgr/index.html
There you can admire two great photos of the results of electromigration across tracks and solder resist on printed circuit boards, plus lots of interesting relevant information. Clearly, humidity and voltage gradients at small distances are a bad combination!!

Good ceramic filter reviving to all of you!! 

01 July 2010

Latest developments on the IC-7000 amplifier issue

After receiving an e-mail from Jan, DG3FDM (thanks, Jan!), who points out a mistake I made with the position of the anti-parasitic resistor on the PCB (it's only in the DC bias path at the point I connected it, since the RF signal comes from the other side of the PCB, after the PCB position I indicated - the schematic is OK though), I think it's time to present the latest information I have since gathered on the matter.

In just a few words, do ONLY the heat sink mod of the 23rd April 2010 post! The other mods don't hurt anything, but apparently they are not needed. (However, if you have already done them, just leave them in place!)

After observing the behaviour of the IC-7000s I have performed the mods on for some time, I think I have finally spotted the true reason for the malfunction, and apparently it is NOT the suspected parasitic oscillation of the DRIVER unit!

Let me explain: After performing the second part of the "stabilising" mods (16 March 2010), that IC-7000 disappointingly blew the driver again in about 15 days. Then, after again replacing the driver, I also installed the heat sinks on the pre-driver and pre-pre-driver transistors, which I noticed that became extremely hot after pressing the PTT for a few seconds (and then I posted the "last" 7000 mods, on April 23rd).

That particular IC-7000 has been in constant use on all bands since then, without any more problems. It had quickly blown the driver transistor three times before the installation of the heat sinks!

There is also more evidence coming from my FT-817, which also stopped blowing the (MOSFET) final amplifier only after I simultaneously installed similar improvised heat sinks on the (also terribly overheating) DRIVER  transistors, along with the anti-parasitic resistors!! I have concluded that the heat sinks solved the  problem, NOT the anti-parasitic resistors! (I had included the heat sink mod in the FT-817 paper in www.mods.dk, of course without recognizing its importance then, I just thought it would be nice to cool those poor flaming transistors).

So there is very strong evidence that the anti - parasitic resistors and other such measures are actually not needed, the overheated pre-driver transistors caused the problem in both the IC-7000 and the FT-817 (and perhaps in many more transceivers with overheating driver and pre-driver stages)!

With the pre-driver heat sinks, the problem seems to have immediately stopped for good in both transceivers (my 817 has been working OK for more than 18 months now, in mostly portable operation).

So, please install the heat sinks only.  The other anti-parasitic mods don't hurt the transceiver in any way, on the contrary, they may remain in place if already performed, but my current results show that the only mod that needs to be done is that of the heat sinks on the severely overheating pre-drivers.